Modulated-carrier signal-translating system



Patented Jan. 23, 1940 PATENT. OFFICE MODULAT'ED-CARRIERSIGNAL-TRANSLAT- ING SYSTEM Harold M. Lewis, Great Neck, Y., assignorto- Hazeltine Corporation, a corporation of Delaware Application January10, 1939, Serial No. 250,130

11 Claims.

This invention relates to modulated-carrier signal-translating systemsand is particularly concerned with systems of the single-sideband typeutilizing a carrier wave and only one sideband of modulation frequenciesor a carrier wave and a band of modulation frequencies excluding atleast a major part of one of the sidebands of modulation frequencies.

In accordance with conventional practice in modulated-carriersignal-translating systems, at the transmitter there is developed ahigh-frequency carrier wave and a lower-frequency signal waverepresenting the information to be communicated. The signal wave isimpressed as modulation on the carrier wave and there is therebydeveloped a modulated-carrier wave comprising the carrier-frequencycomponent and upper and lower sideband modulation-frequency componentshaving frequencies equal, respectively, to the sums of the carrierfrequency and each signal frequency, and to the difierences between thecarrier frequency and each signal frequency. While the carrier-frequencycomponent and each modulation sideband alone include componentsrepresentative of all the information being transmitted, at thereceiver, the carrier wave and both sidebands are generally detected toderive the originalmodulation signal for reproduction.

For various reasons it is frequently desirable to transmit the carrierwave and only one of the modulation sidebands. In television systems,for

example, where the modulation-signal frequencies extend over anexceedingly wide range,'of the order of four megacycles, the modulatedcarrier requires a relatively large portion of the limited part of thefrequency spectrum 'which is available. For the purpose of economy withrespect to the available part of the frequency spectrum, therefore, ithas been proposed to develop and transmit single-sideband signals.

While in some systems single-sideband signals have been transmitted,received, and. reproduced with satisfactory results, it is well knownthat such systems are both theoretically and practically imperfect. Aperfect replica of the modulation signal cannot be derived from asinglesideband and carrier-wave under all conditions. This may beexplained as follows: In detecting the usual modulated-carrier wave withits two modulation sidebands, the detector derives from the signal inputthereto beat notes between the carrier wave and each of thesideband-frequency components. Corresponding upper and lowersideband-frequency components produce with the carrier wave the samebeat notes which represent the modulation signal and they are cumulativein the output circuit ofthe detector. The detector also produces beatnotes between each sideband-frequency component and all others which donot-represent the modulation signal but constitute undesirabledistortion components. Where-both sidebands are present, however, theundesired beat notes due-toone sideband cancel out those due to theother sideband. Where, however, the signal input to the detectorcomprises the carrier wave and only one modulation sideband, theseundesired beat notes donot cancel but are present in the signal outputand tend to destroy the fidelity of reproduction of the desiredmodulation components.

One manner of minimizing such undesirable results insingle-sideband'systems has been to develop and transmit a signal havinga carrier wave of a substantially greater amplitude than any of thesideband-frequency components. Consequently, in such systems, the beatnotes between the sideband-frequency components themselves are ofsubstantially lesser amplitudes than the beat notes between thesideband-frequency components and the carrier wave, and the undesirableeffects are reduced.

In certain single-sideband systems, however, it is desirable thatcertain sideband-frequency components be of such amplitude relative, tothat of the carrier wave that the beat notes therebetween are ofappreciable Values relative to the useful beat notes, renderingdetection thereof highly unsatisfactory. For example, in televisionsystems, where there is a sharp contrast between adjacent incremental"areas of the image being transmitted, the boundary between these areasis represented in the signal by high-frequency modulation componentscomprising a-relatively great percentage of modulation so that the beatnotes between such sideband-frequency components are sufficiently greatto impair the reproduction of a single-sideband signal under certainconditions.

-It may be stated generally, therefore, that only where the signalimpressed upon the detector comprises the carrier wave and both itsmodulation sidebands can the exact replica of the original modulationsignal be derived under all conditions. For thisreason, it has beenconventional practice to utilizedouble sideband; signals except wherespecial considerations render expedient the compromise of utilizingsingle-sideband signals.

It is an object of the present invention to provide a modulated-carriersignal-translating system whereby there may be derived and utilized,from a signal comprising a carrier wave and a modulation-frequency bandexcluding at least a major portion of one of the sidebands ofmodulation, the excluded modulation-frequency components. I

It is another object of the invention to provide a modulated-carriersignal-translating system whereby there may be derived from an availablesingle-sideband signal the modulation-frequency components correspondingto the sideband components which are missing from the available signal.

It is a further object of the invention to provide a modulated-carriersignal-translating system whereby benefits incident to both conventionalsingle-sideband transmission and doublesideband reception maybe'obtained.

In accordance with the present invention, there is provided amodulated-carrier signaltranslating system comprising an input circuitadapted to have a signal impressed thereon comprising a carrier wave anda modulation-frequency band excluding at least a major portion of one ofthe modulation-frequency sidebands. Non-linear signal translating meansare coupled to the input circuit for distorting the impressed signal toderive therefrom signal-frequency components corresponding to theexcluded portion of said sideband. An output circuit is coupled to thesignal-translating means for utilizing the derived modulation-frequencycomponents.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing and itsscope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a schematic diagram of a completesuperheterodyne radio receiver including a signaltranslating systemembodying the present invention, while Fig. 2 is a circuit diagram of amodified form of the signal-translating system.

Referring now more particularly to Fig. 1, the system there illustratedcomprises a receiver of the superheterodyne type including an antennaI0, I I connected to a radio-frequency amplifier I2 to which there isconnected in cascade, in the order named, an oscillator-modulator I3, anintermediate-frequency amplifier I4, a signaltranslating systemindicated generally at I5 and embodying the present invention, adetector I6, a modulation-signal amplifier I1 and a signalreproducingdevice I8. Where the receiver of Fig. 1 comprises a television receiver,the signalreproducing device may include scanning-wave generators, and acathode-ray reproducing tube. Referring briefly to the general operationof the system just described, a signal intercepted by the antenna I0, Il is selected and amplified in the radio-frequency amplifier l2 andsupplied to the oscillator-modulator I3, wherein it is converted into anintermediate-frequency signal which, in turn, is selectively amplifiedin intermediate-frequency amplifier I4 and delivered to theintermediate-frequency signal-translating system I5. The system I5operates in accordance with the present invention, as will be presentlyfully explained, to deliver an intermediatefrequency signal to thedetector [6 wherein the modulation signal is derived and from which itis supplied to the modulation-signal amplifier I I which amplifies thissignal and supplies it to the signal-reproducing device l8 forreproduction. It is to be noted that the receiver of Fig. 1 is adaptedfor the reception of a single-sideband signal, comprising a carrier waveand a modulation-frequency band excluding at least a major portion ofone of the modulation-frequency sidebands. The intermediate-frequencyamplifier I4, therefore, isdesigned to pass the intermediatefrequencycomponents corresponding to the received signal and developed therefromby the oscillator-modulator l3.

Referring now more particularly to the por tion of the receiver of Fig.1 which embodies the present invention, the intermediate-frequencysignal-translating system I5 comprises an input v circuit I9, anon-linear signal-translating device 25 and an output circuit 2i. Theinput circuit is preferably a band-pass selector comprising adouble-tuned transformer 22 having its primary circuit coupled to theoutput circuit of the intermediate-frequency amplifier I4 as shown. Theresponse characteristic of the input circuit or selector I9 is indicatedby curve R1 shown above the selector in Fig. 1, the mean frequency ofthe selector being indicated at o and the location of the carrier beingindicated at 0. While any suitable elements may be utilized for thenonlinear signal-translating device 20, such as properly biasedmulti-grid tubes, in the arrangement shown a pair of diodes 23 and 24are connected in parallel in opposite senses with suitable biasingbatteries 25 and 26, respectively, included in their cathode circuits.The device 20 interconnects the high-potential terminals of the inputcircuit I9 and the output circuit 2| which have a common low potentialterminal. The response or repeating ratio characteristic of the device20 is illustrated by curve R2 shown above the de: vice in Fig. 1. Moreparticularly, the upper and lower halves 1'1 and T2 of curve R2represent the characteristics of the biased diodes 23 and 24,respectively, taken individually. For the purpose of clarity the curves1'1 and r2 have been shown Withthe same zero base line but extending inopposite senses, since the diodes are connected in opposite senses andpass half-cycles of the signal of opposite polarities. It will beapparent that the device 20 has a non-linear response characteristicsuch that the signal translated thereby is distorted in a predeterminedmanner, as presently will be more fully explained.

The output circuit of the signal-translating system is preferably aband-pass selector of the dead-end filter type designed to pass a Wideband of frequencies. This circuit may comprise two double-tunedtransformers 21 and 28 and a terminating resistor 29 interposedtherebetween. The detector I6 is connected across the secondary circuitof transformer 21. The response characteristic of the output circuit orselector 2| is illustrated by curve R3 shown above the selector in Fig.1 from which it is seen that the mean frequency of the pass band of theselector is the same as the frequency of intermediate-carrier wave andthe selector is uniformly responsive over both the modulation sidebands.

In the operation of the signal-translating system l5, a single-sidebandsignal is impressed upon the input circuit [9 and is passed thereby withsubstantially uniform response over the single modulation-frequencysideband, as illustrated by curve R1. ,by the device 20 with anon-linear signal-repeating ratio, as illustrated by curve R2, so as toeffect a predetermined distortion of the wave form of the signal. Byvirtue of the non-linear characteristic of the device I5 it not onlypasses the impressed orinput-signal frequency components but also itfurther functions to derive fromthe input signal the excluded or missingsidebandfrequency components. The complete signal including the carrierwave and both the modulation-frequency sidebands are thus translated tothe output circuit 2| and this circuit as just explained having aband-pass characteristic illustrated by curve R3 passes all thefrequency come The signal is then translated v E1 cos mt -(1) wherew1=21rf1 and the modulation-frequency wave is represented .by

W .Ez cos wzt (2) where r w2=21rf2 and the modulated-carrier wave isrepresented by the formula:

em= E1E1+Ez cos was] cos at which when expanded gives:

If the lower-frequency sideband is suppressed,

the equation of the resultant carrier with only the upper-frequencysideband is:

v e=E COS w t+E /2 COS (w +w2)f I (4) Now the non-linear device 20 hasan input voltage-output current characteristic which is represented bythe general equation:

rt- 11 a e a e a e a e etc. (5)

It can be shown that only the odd power terms contribute to reproducingthe sideband-frequency component missing from Equation 4. Thecharacteristic of the non-linear device 28 of the present invention isof a form such that the even power terms cancel and its general equation(which also defines curve R2 of Fig. 1 of the drawings) is:

' The term the represents the linearly repeated component of the appliedsignal while each of the other terms contribute to the production of themissing sideband-frequency components. For

' simplicity it will be shown that this is true for the termase byrewriting Equation 4 as follows:

. a a e =a [E cos t+%*cos (w +wg)t] (7) This is of the form v +.V) Y+ Y+.V

and it can be shown that only the second term is-of interest asrepresenting frequencies lower than thecarrier. Hence A Since E1 isconstant, the last term may be written in the form:

KE .cos (w w )t (10) which is the third term of Equation 3 representingthe suppressed sideband component.

nected in cascade, as shown. I may comprise a double-tuned transformer33 preferably having its secondary circuit shunted shownthat, incase theupper or opposite sideband is suppressed and only the lower sideband istransmitted, the-upper sideband may be derived by the apparatus-of thepresent invention. Thus,

in accordance with the system of the present in-' vention, when a signalisimpressed thereon comprising a carrier wave and a modulation- In amanner similar to the above, it can be frequency sideband excluding one,or atleast a" .major portion of one, of the modulationfrequencysidebands, the excluded sidebandfreqenc'y components are derived fromthissignali and reinserted in the output signal.

It can be shown "that the device 29 develops from the input signal, inaddition to the desired 1 suppressed modulation sideband, modulation.

products corresponding to the higher order terms of its characteristicequation which constitute carrier waves of higher frequencies togetherwith both their respective modulation-frequency sidebands.

their modulation sidebands. This may be readily accomplished by simplyadjusting the output circuit 2| to be uniformly responsive over thedesired. higher band of frequencies. Moreover, while the particulardevice 20 is so arranged that its characteristic equation includes onlyodd power terms, it will be appreciated that it is within the scope ofthe present invention to so design the devicev 2.0 that itscharacteristic equation includes only the even power terms or includesboth the evenand odd powerterms, and

to design the output circuit so that it is responsive to themodulated-carrier waves derived under these conditions, which wavesinclude the carrier wave and its sideband frequency components atvarious harmonics of the original signal-input frequencies.

In Fig. 2 thereis. illustrated a signal-translat-.

ing system a which is a modified form of the system I5 of Fig. 1 andmaybe substitutedthere for in the receiver of Fig. 1. The'system I511,comprises an input circuit lga, a' signal-translating section Mia, andan output circuit 2Ia..' The input circuit comprisesadouble-tuned'transformer '38 having its primary circuit adapted to becoupled to the outputcircuit of the intermediate- .'frequency amplifierl4. 1

The section a comprisestwo separate signal,- translating channels. Onechannel includes a In certain cases it may be desired to utilize thehigher-frequency carrier waves and vacuum-tube amplifier 3!, of thetriode or other suitable type, and a band-pass selector 3[2 conby adamping resistor "33b and so adjustedas to have a band-passcharacteristic as illustrated by,

curve R4, its mean frequency'being located at substantially thecenterfrequency cf thesingle sideband of frequencies of the input signalas indicated at o, and the carrier frequency being located near one edgeof the pass band.

The other channel of the section Zita comprises a vacuum-tube amplifier31a and selector -3-2a which may be substantially similar to theamplifier 3| and selector 32, respectively. 'Here, however, anintermediate tap on the secondary circuit of .the double-tunedtransformer 33a is grounded and its opposite terminals are 'connected,by way of non-linear signal-translating devices, for example, diodes 36and 35 with suitable biasing batteries 35 and'3lin series therewith, tothe opposite terminalsof the primary circuit of a double-tunedtransformer 38: The

The selector 32 latter transformer constitutes a band-pass selector 39similar to the selectors 32 and 32a. An intermediate tap of the primaryWinding of the transformer 38 is grounded. The diodes 34 and i 35function similarly to the diodes 23 and 24 of Fig. ,1, their generalrepeating ,ratio characteristic taken together being the same as thatillustrated by curve R2 of Fig. 1. Being connected to opposite terminalsof the selectors 32a and 33, the diodes 33, 33 are connected in the samesense.. Suitable adjustment of the batteries 36 and 3'! serveseffectively to shift the curves T1 and r2 laterally in opposite sensesso that the desired over-all characteristic most suitable for developingthe missing sideband-frequency components from the input signal may beobtained. The selector 39 has a band-pass characteristic illustrated by,curveRs similar to that of the selector 32, but having a mean frequencysubstantially corresponding to the mean frequency of the missingsideband. derived by the nonlinear devices 34 and 35 so that it favorsthese derived frequencies. Also, the carrier frequency is located nearthe opposite edge of the pass band from that of selector 32.

The output circuit, Zia may comprise a pair of combining amplifiers 39and 33 having their input electrodes coupled across the secondarycircuits of the transformers 33 and 38 by way of biasing batteries 4|andv 42, respectively. A

common output circuit comprising a resistor 33 is provided for the tubes33 and 43, across which the input circuit of the detector .15 may beconnected. This output circuit is adapted to translate the signal andboth its modulation-frequency sidebands. The biasing batteries 4!, 42are adjusted so that the relative gains of the amplifiers 39 and Allcompensate for any difference in-the amplitudes of the signals as trans-.lated by the two channels.

The operation of the system IE4: is believed obvious from the abovedescription. Briefly, the

single-sideband signal is supplied to the inputcircuit 19a and thechannel including the tube ,3I andselector 32 amplifies and translatesthe carrier .wave and its sideband-frequency components to the outputcircuit 21a. In the other channel, however, while the input signal isalso amplified and translated by the tube 3m and selector 32a, thediodes 34 and 35 operate with a repeating ratio characteristicequivalent to that illustrated by curve R2 of Fig. 1, that is, with anon-linear repeating ratio such as to distort the input signal andderive therefrom the missing side-band-frequency components. Thesederived frequency components are selectively translated by the selector39 to the output circuit and the latter operates to combine them withthe carrier wave and other sideband-frequency components translated bythe channel 3|, 32. There is thus developed in the output circuit thecomplete signal including the carrier wave and both modulation-frequencysidebands, which signal may thereafter be delivered to the detector fordemodulation.

While there have been described what are at present considered thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from this invention, and, therefore, it isaimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of this invention.

What is claimed is:

1. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and a modulation-frequency band excluding at least a major portion of one of themodulation sidebands, non-" linear signal-translating means coupled tosaid input circuit for distorting said impressed signal to derivetherefrom signal components corresponding to said excluded portion ofsaid sidee band, and an output circuit coupled to said sig-.

nal-translating means for utilizing said derived signal components.

2. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, nonlinear signal-translating means coupled tosaid input circuit for distorting said impressed signal to derivetherefrom said excluded portion of said sideband, and an output circuitcoupled to said signal translating means for translating said carrierwave and both its sidebands of modulation frequencies including saidderived excluded portion of said other sideband.

3. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, nonlinear signal-translating means coupled tosaid input circuit and having a non-linear signalrepeating ratio fortranslating said impressed signal with a predetermined distortion toderive therefrom said excluded portion of said sideband, and an outputcircuit coupled to said signaltranslating means for utilizing saidderived signal components. 7

4. A signal translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier Wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, nonlinear signal-translating means coupled tosaid input circuit and having a non-linear signalrepeating ratio fortranslating said impressed signal with a predetermined distortion toderive therefrom said excluded portion of said sideband, and an outputcircuit coupled to said signaltranslating means for translating saidsignalcarrier wave and both its modulation sidebands including saidderived excluded portion of said other sideband.

5. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, said input circuit including a band-passselector adapted to pass said signal with substantially uniformattenuation over said modulation band, non-linear signal-translatingmeans coupled to said input circuit for distorting said impressed signalto derive therefrom said excluded portion of said sideband, and anoutput circuit coupled to said signal-translating means for translatingsaid carrier Wave and both its modulation sidebands including saidderived excluded portion of said other sideband and including aband-pass selector having a substantially uniform responsecharacteristic over said sidebands.

6. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, said input circuit including a band-passselector having a mean frequency at substantially the mean frequency ofsaid band and adapted to pass said signal with uniform attenuation oversaid modulation band, non-linear signal-translating means coupled tosaid input circuit for distorting said impressed signal to derivetherefrom said excluded portion of said sideband, and an output circuitcoupled to said signal-translating means for translating said carrierwave and both its modulation sidebands and including a band-passselector having a mean frequency equal to the carrier-frequency and asubstantially uniformly.

responsive characteristic over said sidebands.

7. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, nonlinear signal-translating means including apair of diodes coupled in opposite senses to said input circuit fornon-linearly translating said impressed signal to effect a predetermineddistortion thereof to derive therefrom said excluded portion of saidsideband, and an output circuit coupled to said signal-translating meansfor utilizing said derived signal components.

8. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, signal-translating means, including a pair ofdiodes coupled in opposite senses to said input circuit, and meansproviding predetermined biases for said diodes, for non-linearlytranslating said impressed signal to effect a predetermined distortionthereof to derive therefrom said excluded portion of said sideband, andan output circuit coupled to said signal-translating means for utilizingsaid derived signal components. I

9. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, a first channel for translating said signal, asecond channel including signal-translating means coupled to said inputcircuit for distorting said impressed signal to derive therefrom saidexcluded portion of said sideband, and a common output circuit for saidchannels for translating said carrier wave and both its modulationsidebands including said derived excluded portion of said othersideband. I

10. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, a

first channel for translating said signal, a second channel includingsignal-translating means coupled to said input circuit for distortingsaid impressed signal to derive therefrom said excluded portion of saidsideband, a common output circuit for said channels for'translatingsaid. carrierv Wave and both its modulation-frequency sidebandsincluding said derived excluded portion of said other sideband, andmeans for adjusting the gain of one of said channels to adjust therelative amplitudes of said directly translated and said derivedsignals.

11. A signal-translating system comprising, an input circuit adapted tohave a signal impressed thereon comprising a carrier Wave and amodulation-frequency band excluding at least a major portion of one ofthe modulation sidebands, a first channel for translating said signalincluding a band-pass selector adapted to pass said signal with uniformresponse over said modulation-frequency band with said carrier Wave atone edge of its pass band, a second channel including signal-translatingmeans coupled to said input circuit for distorting said impressed signalto derive therefrom said excluded portion of said sideband, and aband-pass selector adapted to pass said excluded portion of saidsideband with uniform response thereover and with said carrier atanopposite edge of its pass band, a common output circuit for saidchannels for translating said carrier wave and both itsmodulation-frequency sidebands including said derived excluded portionof said other sideband.

HAROLD M. LEWIS,

